Scintillation crystal including a co-doped sodium halide

ABSTRACT

A scintillation crystal can include a sodium halide that is co-doped with thallium and another element. In an embodiment, the scintillation crystal can include NaX:Tl, Me, wherein X represents a halogen, and Me represents a Group 1 element, a Group 2 element, a rare earth element, or any combination thereof. In a particular embodiment, the scintillation crystal has a property including, for radiation in a range of 300 nm to 700 nm, an emission maximum at a wavelength no greater than 430 nm; or an energy resolution less than 6.4% when measured at 662 keV, 22° C., and an integration time of 1 microsecond. In another embodiment, the co-dopant can be Sr or Ca. The scintillation crystal can have lower energy resolution, better proportionality, a shorter pulse decay time, or any combination thereof as compared to the sodium halide that is doped with only thallium.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation and claims priority under 35U.S.C. § 120 to U.S. Non-Provisional patent application Ser. No.16/163,705, filed Oct. 18, 2018, entitled “Scintillation CrystalIncluding a Co-Doped Sodium Halide,” naming as inventors Kan Yang etal., which is a continuation and claims priority under 35 U.S.C. § 120to U.S. Non-Provisional patent application Ser. No. 15/908,165, filedFeb. 28, 2018, entitled “Scintillation Crystal Including a Co-DopedSodium Halide,” naming as inventors Kan Yang et al., now U.S. Pat. No.10,134,499, which is a continuation and claims priority under 35 U.S.C.§ 120 to U.S. Non-Provisional patent application Ser. No. 15/043,812,filed Feb. 15, 2016, entitled “Scintillation Crystal Including aCo-Doped Sodium Halide, and a Radiation Detection Apparatus Includingthe Scintillation Crystal,” naming as inventors Kan Yang et al., nowU.S. Pat. No. 9,947,427, which claims priority under 35 U.S.C. § 119(e)to U.S. Provisional Patent Application No. 62/116,734, filed Feb. 16,2015, entitled “Scintillation Crystal Including a Co-Doped SodiumHalide, and a Radiation Detection Apparatus Including the ScintillationCrystal,” naming as inventors Kan Yang et al., all of which are assignedto the current assignee hereof and are incorporated by reference hereinin their entireties.

FIELD OF THE DISCLOSURE

The present disclosure is directed to scintillation crystals includingrare earth halides and radiation detection apparatuses including suchscintillation crystals.

BACKGROUND

NaI:Tl is a very common and well known scintillation crystal. Furtherimprovements with NaI:Tl scintillation crystals are desired.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example and are not limited in theaccompanying figures.

FIG. 1 includes an illustration of a radiation detection apparatus inaccordance with an embodiment that can be used in medical imaging.

FIGS. 2 and 3 include illustrations of a radiation detection apparatusin accordance with an embodiment that can be used in drilling or welllogging.

FIG. 4 includes obtained for a Sr co-doped and standard sample whenexposed to gamma radiation at an energy of 662 keV at room temperatureand a pulse integration of 1 microsecond.

FIG. 5 includes obtained for a La co-doped and standard sample whenexposed to gamma radiation at an energy of 662 keV at room temperatureand a pulse integration of 1 microsecond.

FIG. 6 includes a plot of relative light yield as a function of energyfor Sr co-doped, Ca co-doped, and standard samples.

FIG. 7 includes a plot of scintillation light yield versus pulse decaytime for NaI:Tl crystals co-doped with different concentrations of Ca.

Skilled artisans appreciate that elements in the figures are illustratedfor simplicity and clarity and have not necessarily been drawn to scale.For example, the dimensions of some of the elements in the figures maybe exaggerated relative to other elements to help to improveunderstanding of embodiments of the invention.

DETAILED DESCRIPTION

The following description in combination with the figures is provided toassist in understanding the teachings disclosed herein. The followingdiscussion will focus on specific implementations and embodiments of theteachings. This focus is provided to assist in describing the teachingsand should not be interpreted as a limitation on the scope orapplicability of the teachings.

Group numbers corresponding to columns within the Periodic Table of theelements use the “New Notation” convention as seen in the CRC Handbookof Chemistry and Physics, 81^(st) Edition (2000-2001).

The term “rare earth” or “rare earth element” is intended to mean Y, Sc,and the Lanthanoid elements (La to Lu) in the Periodic Table of theElements.

As used herein, the terms “comprises,” “comprising,” “includes,”“including,” “has,” “having,” or any other variation thereof, areintended to cover a non-exclusive inclusion. For example, a process,method, article, or apparatus that comprises a list of features is notnecessarily limited only to those features but may include otherfeatures not expressly listed or inherent to such process, method,article, or apparatus. Further, unless expressly stated to the contrary,“or” refers to an inclusive-or and not to an exclusive-or. For example,a condition A or B is satisfied by any one of the following: A is true(or present) and B is false (or not present), A is false (or notpresent) and B is true (or present), and both A and B are true (orpresent).

The use of “a” or “an” is employed to describe elements and componentsdescribed herein. This is done merely for convenience and to give ageneral sense of the scope of the invention. This description should beread to include one or at least one and the singular also includes theplural, or vice versa, unless it is clear that it is meant otherwise.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. The materials, methods, andexamples are illustrative only and not intended to be limiting. To theextent not described herein, many details regarding specific materialsand processing acts are conventional and may be found in textbooks andother sources within the scintillation and radiation detection arts.

A scintillation crystal can include a sodium halide that is co-dopedwith thallium and another element. The co-doping can improve energyresolution, proportionality, light yield, decay time, another suitablescintillation parameter, or any combination thereof. In an embodiment,the scintillation crystal can include NaX:Tl, Me, wherein X represents ahalogen, and Me represents a Group 1 element, a Group 2 element, a rareearth element, or any combination thereof. The selection of a particularco-dopant may depend on the particular scintillation parameters that areto be improved. As used in this specification, co-doping can include twoor more different elements (Tl and one or more other elements), andco-dopant refers to the one or more dopants other than Tl.

With respect to the composition of the sodium halide, X can be I or acombination of I and Br. When X is a combination of I and Br, X caninclude at least 50 mol %, at least 70 mol %, or at least 91 mol % I. Inan embodiment, Tl can have a concentration of at least 1×10⁻⁴ mol %, atleast 1×10⁻³ mol %, or at least 0.01 mol %, and in another embodiment,Tl has a concentration no greater than 5 mol %, no greater than 0.2 mol%, or no greater than 0.15 mol %. In a particular embodiment, Tl has aconcentration in a range of 1×10⁻⁴ mol % to 5 mol %, 1×10⁻³ mol % to 0.2mol %. In a more particular embodiment, the Tl has a concentration in arange of 0.03 mol % to 0.15 mol % exhibits good performance, such ashigh light yield and good energy resolution.

In an embodiment when the co-dopant is a Group 1 element or a Group 2element, the dopant concentration of the Group 1 element or the Group 2element is at least 1×10⁻⁴ mol %, at least 1×10⁻³ mol %, or at least0.01 mol %, and in another embodiment, the concentration is no greaterthan 5 mol %, no greater than 0.9 mol %, or no greater than 0.2 mol %.In a particular embodiment, the concentration of the Group 1 element orGroup 2 element is in a range of 1×10⁻⁴ mol % to 5 mol %, 1×10⁻³ mol %to 0.9 mol %, or 0.01 mol % to 0.2 mol %. In an embodiment when theco-dopant is a rare earth element, the dopant concentration of the rareearth element is at least 5×10⁻⁴ mol % or at least 8×10⁻⁴ mol %, and inanother embodiment, the dopant concentration is no greater than 0.5 mol%, 0.05 mol %, or 5×10⁻³ mol %. All of the preceding dopantconcentrations are for dopant concentrations in the crystal.

In a particular embodiment, the scintillation crystal ismonocrystalline. The concentration of dopants in the crystal may or maynot be different from the concentrations of dopants in the melt fromwhich the scintillation crystal is formed. The concentrations of Tl andGroup 1 and Group 2 co-dopants in a melt in forming the crystal caninclude any of the values as previously described. Rare earth elements,and particular, La and heavier elements are relatively large as comparedto Na atoms, and thus, a significantly lower concentration of a rareearth element can result. Thus, in one embodiment, for a rare earthelement, the dopant concentration in the melt may be higher, such as atleast 1×10⁻³ mol %, at least 5×10⁻³ mol %, or at least 0.01 mol %. Afterreading this specification in its entirety, skilled artisans will beable to determine the amounts of dopants to be in a melt to achieveddesired dopant concentrations in a scintillation crystal afterconsidering segregation coefficients for such dopants.

When selecting a co-dopant, different considerations may determine whichparticular elements are better suited for improving particularscintillation parameters as compared to other elements. The descriptionbelow is to be used as general guidance and not construed as limitingparticular scintillation parameters to particular co-dopants.

In an embodiment, the wavelength of emission maximum for NaX:Tl, Mescintillation crystal may be kept relatively the same as NaX:Tl, so thatthe quantum efficiency of a photosensor that is or will be coupled tothe scintillation crystal is not significantly changed. For radiation ina range of 300 nm to 700 nm, NaI:Tl has an emission maximum between 415nm and 420 nm when the scintillation crystal is exposed to gammaradiation having an energy of 60 keV. In an embodiment, for radiation ina range of 300 nm to 700 nm, the co-doped scintillation crystal has anemission maximum at a wavelength of at least 400 nm, at least 405 nm, orat least 410 nm when the scintillation crystal is exposed to gammaradiation having an energy of 60 keV, and in another embodiment, theco-doped scintillation crystal has an emission maximum at a wavelengthno greater than 430 nm, no greater than 425 nm, or no greater than 420nm when the scintillation crystal is exposed to gamma radiation havingan energy of 60 keV. In a particular embodiment, the co-dopedscintillation crystal has an emission maximum at a wavelength in a rangeof 400 nm to 430 nm, 405 nm to 425 nm, or 410 nm to 420 nm when thescintillation crystal is exposed to gamma radiation having an energy of60 keV.

Many Group 1, Group 2, and rare earth elements may be a co-dopant inNaX:Tl, Me without significantly affecting the wavelength of theemission maximum as compared to NaX:Tl. Co-doping with a rare earthelement can increase light yield. Some of the rare earth elements may bebetter suited for use without affecting the emission maximum. Forexample, co-doping with Sc, Y, La, Lu, and Yb are less likely to affectsignificantly the wavelength of the emission maximum. Eu may shift theemission maximum to approximately 440 nm, and such a shift may cause thequantum efficiency to decrease or a different photosensor with a higherquantum efficiency at 440 nm to be selected. Co-doping with other rareearth elements that are more likely to cause a significant shift in thewavelength of the emission maximum may include Sm, Pr, Nd, and Tb.

Scintillation crystals that include co-doping with both a divalent metalelement (e.g., a Group 2 element) and a rare earth element, in additionto Tl, can provide improved performance. The combination of the divalentmetal element and the rare earth element as co-dopants in thescintillation crystal can have improved (lower) energy resolution,improved (faster) pulse decay time, and suppress defect color centers.Particular embodiments include NaX:Tl co-doped with Sr and Y or co-dopedwith Ca and Y.

Energy resolution (also called pulse height resolution or PHR) isimproved as the energy resolution can be smaller for the co-dopedscintillation crystals. Unless specified otherwise, the industrystandard for determining energy resolution is to expose a scintillationcrystal to ¹³⁷Cs, which emits gamma radiation at an energy of 662 keV.The testing is performed at room temperature, such as 20° C. to 25° C.,and 22° C. in a particular testing method. The energy resolution is thefull width at the half maximum of the 662 keV peak divided by the peakheight. Energy resolution is typically expressed as a percentage. Energyresolution may be determined over an integration time, also referred toas shaping time. Good energy resolution at a relatively shortintegration time corresponds to a shorter time to correctly identify aradiation source.

In an embodiment, the scintillation crystal has an energy resolutionless than 6.4%, less than 6.2%, or less than 6.0% when measured at 662keV, 22° C., and an integration time of 1 microsecond. In anotherembodiment, the scintillation crystal has an energy resolution less than6.1%, less than 6.0%, or less than 5.9% when measured at 662 keV, 22°C., and any integration time in a range of 1 microsecond to 4microseconds. Although no lower limit for the energy resolution is knownat this time, the scintillation crystal may an energy resolution of atleast 0.1% at any integration time in a range of 1 microsecond to 4microseconds. As a basis for comparison, NaI:Tl has an energy resolutiongreater than 6.2% at integration times of 1, 2, and 4 microseconds.

Group 2 elements are particularly well suited in achieving good energyresolution. Ca and Sr are particularly well suited for achieving lowenergy resolution. NaI:Tl, Sr can achieve an energy resolution of 5.3%at an integration time of 1 microsecond, and NaI:Tl, Ca can achieve anenergy resolution of 5.4% at an integration time of 1 microsecond. Evenlower energy resolutions may be achieved with further optimization.Compare such energy resolution to NaI:Tl, Eu, which is reported to havean energy resolution of greater than 6.4% at an integration time of 1microsecond, and greater than 6.2% at an integration time of 4microseconds. In a particular embodiment, the energy resolution is atleast 0.1% at an integration time of 1 microsecond.

Group 1 elements may also improve energy resolution. While not beingbound by theory, the use of relatively larger Group 1 elements that aresubstituted for Na atoms may put the crystal lattice in tension andimprove hole mobility. Thus, K, Rb, and Cs may be used. K has arelatively high concentration of a radioactive isotope, so Rb and Cs maybe better candidates for a co-dopant.

Proportionality has recently received more attention with respect toscintillation crystals. Ideally, light yield is a perfect linearfunction of energy. Thus, at any energy for a particular scintillationcrystal composition, identification of a radiation source may be easierand be performed with more confidence as a plot of light yield vs.energy is a perfectly straight line. In reality, the light yield candeviate from perfect linearity, and such deviation is typically greaterat lower energies. One method to determine proportionality of ascintillation crystal is to determine the light yield at a high energy,such as 2615 keV. In a plot of light yield vs. energy, a straight linegoes from a point corresponding to 0 light yield, 0 energy (0, 0) toanother point corresponding to the light yield at 2615 keV, 2615 keV(LY₂₆₁₅, 2615).

In terms of an equation, the relative light yield at a particular energy(in units of keV), as normalized to the light yield at 2615 keV, is:

${{{relative}\mspace{14mu} {light}\mspace{14mu} {yield}} = \frac{{actual}\mspace{14mu} {light}\mspace{14mu} {yield}}{{predicted}\mspace{14mu} {light}\mspace{14mu} {yield}}},$

where the actual light yield is at the particular energy, and thepredicted light yield is:

${{{predicted}\mspace{14mu} {light}\mspace{14mu} {yield}} = {\frac{{particular}\mspace{14mu} {energy}}{2615\mspace{14mu} {keV}} \times {LY}_{2615}}},$

where LY₂₆₁₅ is the actual light yield at 2615 keV.

An average relative light yield can be obtained by integrating therelative light yield over a particular energy range to obtain anintegrated value, and dividing the integrated value by the particularenergy range.

A relative light yield of 1.00 corresponds to perfect proportionality.As the deviation increases, either higher than 1.00 or less than 1.00,proportionality is worse. For example a relative light yield of 0.98 isbetter than 1.05 because 1.00 is closer to 0.98 than to 1.05.

In an embodiment, at energies in a range of 32 keV to 81 keV, thescintillation crystal has an average relative light yield as normalizedto a light yield at 2615 keV of at least 1.01, at least 1.04, or atleast 1.07. In another embodiment, at energies in the range of 32 keV to81 keV, the scintillation crystal has an average relative light yield asnormalized to a light yield at 2615 keV of no greater than 1.15, nogreater than 1.13, or no greater than 1.11. In a particular embodiment,at energies in the range of a 32 keV to 81 keV, the scintillationcrystal has an average relative light yield as normalized to a lightyield at 2615 keV that is in a range of 1.01 to 1.15, 1.04 to 1.13, or1.07 to 1.11. As a basis for comparison, at energies in the range of 32keV to 81 keV, NaI:Tl has an average relative light yield as normalizedto a light yield at 2615 keV that is over 1.15.

Although not as great, improvement can be seen at intermediate energies.In an embodiment, at energies in a range of 122 keV to 511 keV, thescintillation crystal has an average relative light yield as normalizedto a light yield at 2615 keV of at least 1.01, at least 1.02, or atleast 1.03. In another embodiment, at energies in the range of 122 keVto 511 keV, the scintillation crystal has an average relative lightyield as normalized to a light yield at 2615 keV no greater than 1.07,no greater than 1.06, or no greater than 1.05. In a particularembodiment, at energies in the range of 122 keV to 511 keV, thescintillation crystal has an average relative light yield as normalizedto a light yield at 2615 keV that is in a range of 1.01 to 1.07, 1.02 to1.06, or 1.03 to 1.05. As a basis for comparison, at energies in therange of 122 keV to 511 keV, NaI:Tl has an average relative light yieldas normalized to a light yield at 2615 keV that is approximately 1.08.

Group 2 elements are good in achieving proportionality. Ca and Sr areparticularly well suited for achieving proportionality closer to 1.00for relative low and intermediate energy ranges. For energies in a rangeof 32 to 81 keV, NaI:Tl, Sr can achieve an average relative light yieldas normalized to a light yield at 2615 keV of 1.09, and NaI:Tl, Ca canachieve an average relative light yield as normalized to a light yieldat 2615 keV of 1.15. For energies in a range of 122 to 511 keV, NaI:Tl,Sr can achieve an average relative light yield as normalized to a lightyield at 2615 keV of 1.04, and NaI:Tl, Ca can achieve an averagerelative light yield as normalized to a light yield at 2615 keV of 1.06.

Scintillation pulse decay time can be decreased with a co-dopant ascompared to the composition without the co-dopant. In an embodiment, thescintillation crystal with the co-dopant has a pulse decay time that isat least 5%, at least 11%, or at least 20% less than a pulse decay timea NaI:Tl crystal when the scintillation crystal and the NaI:Tl crystalare measured at 22° C. and exposed to gamma radiation having an energyof 662 keV. In another embodiment, the scintillation crystal with theco-dopant has a pulse decay time that is no greater than 80%, no greaterthan 65%, or no greater than 50% less than a pulse decay time of NaI:Tlcrystal when the scintillation crystal and the NaI:Tl crystal aremeasured at 22° C. and exposed to gamma radiation having an energy of662 keV. In a particular embodiment, the scintillation crystal with theco-dopant has a pulse decay time that is in a range of 5% to 80%, 11% to65%, or 20% to 50% less than a pulse decay of a NaI:Tl crystal when thescintillation crystal and the NaI:Tl crystal are measured at 22° C. andexposed to gamma radiation having an energy of 662 keV. With respect toactual times, a NaI:Tl crystal may have a pulse decay time ofapproximately 230 ns, and a NaI:Tl, Sr crystal can have a pulse decaytime as low as 160 ns. Similar improvement can occur with NaI:Tl, Ca.

The scintillation crystal can be formed using any one of a variety ofcrystal growing techniques including Bridgman, Czochralski, Kyropoulos,Edge-defined Film-fed Growth (EFG), Stepanov, or the like. The startingmaterials include a sodium halide and halides of the dopants. In anembodiment, the starting materials can include NaI and TlI, anddepending on the co-dopant, the starting material can include any one ormore of Cal₂, SrI₂, Lab₃, YI₃, ScI₃, LuI₃, RbI, CsI, or the like. If arelatively small amount of bromine to be added, any of the dopants (TIor any of the co-dopants) can be a corresponding bromide. If morebromine is desires, NaBr may be substituted for some of the NaI. Afterdetermining a desired composition of the scintillation crystal, skilledartisan will be able to use segregation coefficients for the dopantswith respect to the base material (e.g., NaI) to determine amounts ofstarting materials to use in the melt. Crystal growing conditions canthe same as used in forming NaI:Tl or may have relatively small changesto optimize the crystal formation process.

Any of the scintillation crystals as previously described can be used ina variety of applications. Exemplary applications include gamma rayspectroscopy, isotope identification, Single Positron Emission ComputerTomography (SPECT) or Positron Emission Tomography (PET) analysis, x-rayimaging, oil well-logging detectors, and detecting the presence ofradioactivity. The scintillation crystal can be used for otherapplications, and thus, the list is merely exemplary and not limiting. Acouple of specific applications are described below.

FIG. 1 illustrates an embodiment of a radiation detection apparatus 100that can be used for gamma ray analysis, such as a Single PositronEmission Computer Tomography (SPECT) or Positron Emission Tomography(PET) analysis. In the embodiment illustrated, the radiation detectionapparatus 100 includes a photosensor 101, an optical interface 103, anda scintillation device 105. Although the photosensor 101, the opticalinterface 103, and the scintillation device 105 are illustrated separatefrom each other, skilled artisans will appreciate that photosensor 101and the scintillation device 105 can be coupled to the optical interface103, with the optical interface 103 disposed between the photosensor 101and the scintillation device 105. The scintillation device 105 and thephotosensor 101 can be optically coupled to the optical interface 103with other known coupling methods, such as the use of an optical gel orbonding agent, or directly through molecular adhesion of opticallycoupled elements.

The photosensor 101 may be a photomultiplier tube (PMT), asemiconductor-based photomultiplier, or a hybrid photosensor. Thephotosensor 101 can receive photons emitted by the scintillation device105, via an input window 116, and produce electrical pulses based onnumbers of photons that it receives. The photosensor 101 is electricallycoupled to an electronics module 130. The electrical pulses can beshaped, digitized, analyzed, or any combination thereof by theelectronics module 130 to provide a count of the photons received at thephotosensor 101 or other information. The electronics module 130 caninclude an amplifier, a pre-amplifier, a discriminator, ananalog-to-digital signal converter, a photon counter, a pulse shapeanalyzer or discriminator, another electronic component, or anycombination thereof. The photosensor 101 can be housed within a tube orhousing made of a material capable of protecting the photosensor 101,the electronics module 130, or a combination thereof, such as a metal,metal alloy, other material, or any combination thereof.

The scintillation device 105 includes a scintillation crystal 107 can beany one of the scintillation crystals previously described that arerepresented by a general formula of NaX:Tl, Me, wherein X represents ahalogen, and Me represents a Group 1 element, a Group 2 element, a rareearth element, or any combination thereof. The scintillation crystal 107is substantially surrounded by a reflector 109. In one embodiment, thereflector 109 can include polytetrafluoroethylene (PTFE), anothermaterial adapted to reflect light emitted by the scintillation crystal107, or a combination thereof. In an illustrative embodiment, thereflector 109 can be substantially surrounded by a shock absorbingmember 111. The scintillation crystal 107, the reflector 109, and theshock absorbing member 111 can be housed within a casing 113.

The scintillation device 105 includes at least one stabilizationmechanism adapted to reduce relative movement between the scintillationcrystal 107 and other elements of the radiation detection apparatus 100,such as the optical interface 103, the casing 113, the shock absorbingmember 111, the reflector 109, or any combination thereof. Thestabilization mechanism may include a spring 119, an elastomer, anothersuitable stabilization mechanism, or a combination thereof. Thestabilization mechanism can be adapted to apply lateral forces,horizontal forces, or a combination thereof, to the scintillationcrystal 107 to stabilize its position relative to one or more otherelements of the radiation detection apparatus 100.

As illustrated, the optical interface 103 is adapted to be coupledbetween the photosensor 101 and the scintillation device 105. Theoptical interface 103 is also adapted to facilitate optical couplingbetween the photosensor 101 and the scintillation device 105. Theoptical interface 103 can include a polymer, such as a silicone rubber,that is polarized to align the reflective indices of the scintillationcrystal 107 and the input window 116. In other embodiments, the opticalinterface 103 can include gels or colloids that include polymers andadditional elements.

The scintillation crystal can be used in a well logging application.FIG. 2 includes a depiction of a drilling apparatus 10 includes a topdrive 12 connected to an upper end of a drill string 14 that issuspended within a well bore 16 by a draw works 17. A rotary table,including pipe slips, 18 can be used to maintain proper drill stringorientation in connection with or in place of the top drive 12. Adownhole telemetry measurement and transmission device 20, commonlyreferred to as a measurement-while-drilling (MWD) device, is part of adownhole tool that is connected to a lower end of the drill string 14.The MWD device transmits drilling-associated parameters to the surfaceby mud pulse or electromagnetic transmission. These signals are receivedat the surface by a data receiving device 22. The downhole tool includesa bent section 23, a downhole motor 24, and a drill bit 26. The bentsection 23 is adjacent the MWD device for assistance in drilling aninclined well bore. The downhole motor 24, such as apositive-displacement-motor (PDM) or downhole turbine, powers the drillbit 26 and is at the distal end of the downhole tool.

The downhole signals received by the data reception device 22 areprovided to a computer 28, an output device 30, or both. The computer 28can be located at the well site or remotely linked to the well site. Ananalyzer device can be part of the computer 28 or may be located withinthe downhole tool near the MWD device 20. The computer 28 and analyzerdevice can include a processor that can receive input from a user. Thesignals are also sent to an output device 30, which can be a displaydevice, a hard copy log printing device, a gauge, a visual audial alarm,or any combination thereof. The computer 28 is operatively connected tocontrols of the draw works 17 and to control electronics 32 associatedwith the top drive 12 and the rotary table 18 to control the rotation ofthe drill string and drill bit. The computer 28 may also be coupled to acontrol mechanism associated with the drilling apparatus's mud pumps tocontrol the rotation of the drill bit. The control electronics 32 canalso receive manual input, such as a drill operator.

FIG. 3 illustrates a depiction of a portion of the MWD device 20 withinthe downhole tool 16. The MWD device 20 includes a housing 202, atemperature sensor 204, a scintillation crystal 222, an opticalinterface 232, a photosensor 242, and an analyzer device 262. Thehousing 202 can include a material capable of protecting thescintillation crystal 222, the photosensor 242, the analyzer device 262,or a combination thereof, such as a metal, metal alloy, other material,or any combination thereof. The temperature sensor 204 is locatedadjacent to the scintillation crystal 222, the photosensor 242, or both.The temperature sensor 204 can include a thermocouple, a thermistor, oranother suitable device that is capable of determining the temperaturewithin the housing over the normal operating temperature of the MWDdevice 20. A radiation detection apparatus includes the scintillationcrystal 222 that is optically coupled to the photosensor 242 that iscoupled to the analyzer device 262.

The scintillation crystal 222 has a composition that is well suited forhigh temperature applications, such as greater than 120° C., at least130° C., at least 140° C., at least 150° C., and higher. In anembodiment, the scintillation crystal 222 can be any one of thescintillation crystals previously described that are represented by ageneral formula of NaX:Tl, Me, wherein X represents a halogen, and Merepresents a Group 1 element, a Group 2 element, a rare earth element,or any combination thereof.

In another embodiment, a radiation detection apparatus with thescintillation crystal as described herein may be configured for anotherapplication. In a particular embodiment, the radiation detectionapparatus may be configured for use in prompt gamma neutron activationanalysis. The decreased pulse decay time may allow for a simpler designfor the radiation detection apparatus. In particular, a scintillatorcrystal without the co-dopant may need to be heated (above roomtemperature) to achieve a desired pulse decay time. A heater cancomplicate the design of the radiation detection apparatus and may causeundesired noise. With the co-doping, the pulse decay time may besufficiently fast at room temperature (for example, 22° C.), thus,obviating the need for a heater and simplifying the design.

In a further embodiment, the actual light yield from a scintillationevent can be adjusted to improve energy resolution. The concentration ofa dopant within a scintillation crystal may vary throughout the crystal,and the concentration of the dopant can affect the decay time and thelight yield of the scintillation crystal. Thus, the decay time and lightyield can depend on the location where the gamma ray energy is capturedwithin the scintillation crystal. Because light yield is used todetermine the energy of the interacting gamma ray, there could be avariation in the energy measurement. In other words, the energyresolution would be poor. In a further embodiment, the actual lightyield can be adjusted for the variation in light yield based on thepulse decay time. The method described below is particularly well usedfor a co-dopant, such as Ca, Sr, Ba, La, or any other co-dopant whosedecay time is dependent on the concentration of the co-dopant.

Before the adjustment, an equation correlating pulse decay time to lightyield is generated. Data can be collected for different dopantconcentrations of the co-dopant within a scintillation crystal whendetecting radiation a particular gamma ray source, for example, ¹³⁷Cs,which emits gamma radiation at an energy of 662 keV. A plot of decaytime versus light yield provides a linear relationship between thescintillation light yield and the decay time. The pulse decay time isthe time interval between the moment of the peak photon flux in eachpulse and the moment when photon flux has fallen to a factor of l/e(36.8%) of the peak photon flux. Thus, a least squares fit of datacorresponding to the plot can produce Equation 1 below.

LY _(est) =m*DT+b,  (Equation 1)

where:

LY_(est) is the estimated light yield,

DT is the pulse decay time,

m is the slope of the line, and

b is the y-axis intercept of the line.

As a non-limiting example, FIG. 7 includes a plot of scintillation lightyield versus pulse decay time for NaI:Tl crystals co-doped with Ca(triangle symbols). The data in FIG. 7 was generated from samples inwhich the concentration of Ca in the NaI:Tl crystals is different ineach sample. The ordinate of the plot is scintillation light yield inunits of photons/MeV of gamma ray energy. The abscissa is thescintillation pulse decay time in units of nanoseconds. Referring toFIG. 7, the straight line is a least squares fit to the data, and theequation for this line is

LY _(est)=206.28*DT−8360.3,  (Equation 2)

where:

LY_(est) is the estimated light yield in photons/MeV, and

DT is the pulse decay time in nanoseconds.

If needed or desired, different equations may be generated for differentco-dopants, different radiation sources, or both. The equations would beof the same format as Equation 1 and may have different values for m andb for the different co-dopants, radiation sources, or both.

After the values for m and b for Equation 1 have been generated, actualmeasurements of pulse decay time and light yield for a subsequentscintillation event can be determined. The pulse decay time can be usedto determine the estimated light yield in accordance with Equation 1.

The actually measured light yield can be adjusted by the ratio of thelight yield for a scintillator of the same composition without theco-dopant, that is NaX:Tl, using Equation 3 below.

LY _(adj) =LY _(act) *LY _(std) /LY _(est),  (Equation 3)

where:

LY_(adj) is the adjusted light yield,

LY_(est) is the light yield as measured,

LY_(std) is the light yield of a scintillation crystal without aco-dopant, for example NaI:TI, and

LY_(est) is the estimated light yield calculated based on the measuredpulse decay time.

LY_(std) can have a previously known value or may be calculated usingthe pulse decay time for a scintillation crystal without a co-dopant.For NaI:Tl, the pulse decay time can be 230 ns, and Equation 2 can beused to determine LY_(std).

The adjustment allows the light yield of each individual pulse to bescaled by a compensation factor to reduce the variation in the measuredlight yield within the same crystal or between different crystals havingdifferent co-dopant content. This process can improve the energyresolution of the crystal. Accordingly, a radiation source may beidentified more quickly and accurately, as compared to using the actualmeasurement without the adjustment.

Many different aspects and embodiments are possible. Some of thoseaspects and embodiments are described herein. After reading thisspecification, skilled artisans will appreciate that those aspects andembodiments are only illustrative and do not limit the scope of thepresent invention. Embodiments may be in accordance with any one or moreof the embodiments as listed below.

Embodiment 1

A scintillation crystal comprising NaX:Tl, Me, wherein:

X represents a halogen;

Me represents a Group 1 element, a Group 2 element, a rare earthelement, or any combination thereof;

each of Tl and Me has a dopant concentration of at least 1×10⁻⁵ mol %;and

the scintillation crystal has a property including:

for radiation in a range of 300 nm to 700 nm, an emission maximum at awavelength no greater than 430 nm;

an energy resolution less than 6.4% when measured at 662 keV, 22° C.,and an integration time of 1 microsecond; or

a pulse decay time that is less than another scintillation crystal thathas a composition of NaX:Tl.

Embodiment 2

The scintillation crystal of Embodiment 1, wherein Me is a rare earthelement.

Embodiment 3

The scintillation crystal of Embodiment 2, wherein Me is La, Sc, Y, Lu,Yb, or any combination thereof.

Embodiment 4

The scintillation crystal of Embodiment 2 or 3, wherein Me has a dopantconcentration of at least 5×10⁻⁴ mol % or at least 8×10⁻⁴ mol %.

Embodiment 5

The scintillation crystal of any one of Embodiments 2 to 4, wherein Mehas a dopant concentration no greater than 0.9 mol %, 0.05 mol %, or5×10⁻³ mol %.

Embodiment 6

The scintillation crystal of Embodiment 1, wherein Me is a Group 1element.

Embodiment 7

The scintillation crystal of Embodiment 1, wherein Me is a Group 2dopant.

Embodiment 8

The scintillation crystal of Embodiment 7, wherein Me is Ca, Sr, or anycombination thereof.

Embodiment 9

A scintillation crystal comprising NaX:Tl, Sr, wherein:

X represents a halogen; and

each of Tl and Sr has a concentration of at least 1×10⁻⁵ mol %.

Embodiment 10

The scintillation crystal of Embodiment 9, wherein the scintillationcrystal has a greater light yield as compared to a NaI:Tl crystal whenthe scintillation crystal and the NaI:Tl crystal are measured at 22° C.

Embodiment 11

A scintillation crystal comprising NaX:Tl, Ca, wherein:

X represents a halogen;

each of Tl and Ca have a concentration of at least 1×10⁻⁵ mol %; and

the scintillation crystal has a greater light yield, a shorter pulsedecay time, or both as compared to a NaI:Tl crystal when thescintillation crystal and the NaI:Tl crystal are measured at 22° C.

Embodiment 12

A scintillation crystal comprising NaX:Tl, Me²⁺, and RE, wherein:

X represents a halogen;

Me²⁺ represents a divalent metal element;

RE represents a rare earth element; and

each of Tl and Sr has a concentration of at least 1×10⁻⁵ mol %.

Embodiment 13

The scintillation crystal of Embodiment 12, wherein Me²⁺ is Ca, Sr, orany combination thereof; and RE is La, Sc, Y, Lu, Yb, or any combinationthereof.

Embodiment 14

A scintillation crystal comprising NaX:Tl, Me, wherein:

X represents a halogen;

Me represents a Group 1 element and has a concentration in a range of1×10⁻⁵ mol % to 9 mol %.

Embodiment 15

The scintillation crystal of Embodiment 6 or 14, wherein Me is Rb, Cs,or any combination thereof.

Embodiment 16

The scintillation crystal of any one of Embodiments 6 to 15, wherein theGroup 1 element, the Group 2 element, Sr, Ca, Me²⁺ or RE has aconcentration of at least 1×10⁻⁴ mol %, at least 1×10⁻³ mol %, or atleast 0.01 mol %.

Embodiment 17

The scintillation crystal of any one of Embodiments 6 to 16, wherein theGroup 1 element, the Group 2 element, Sr, Ca, Me²⁺ or RE has aconcentration no greater than 5 mol %, no greater than 0.9 mol %, or nogreater than 0.2 mol %.

Embodiment 18

The scintillation crystal of any one of Embodiments 6 to 17, wherein theGroup 1 element, the Group 2 element, Sr, Ca, Me² or RE has aconcentration in a range of 1×10⁻⁴ mol % to 5 mol %, 1×10⁻³ mol % to 0.9mol %, or 0.01 mol % to 0.2 mol %.

Embodiment 19

The scintillation crystal of any one of the preceding Embodiments,wherein Tl has a concentration of at least 1×10⁻⁴ mol %, at least 1×10⁻³mol %, or at least 0.01 mol %.

Embodiment 20

The scintillation crystal of any one of the preceding Embodiments,wherein Tl has a concentration no greater than 5 mol %, no greater than0.9 mol %, or no greater than 0.2 mol %.

Embodiment 21

The scintillation crystal of any one of the preceding Embodiments,wherein Tl has a concentration in a range of 1×10⁻⁴ mol % to 5 mol %,1×10⁻³ mol % to 0.9 mol %, or 0.01 mol % to 0.2 mol %.

Embodiment 22

The scintillation crystal of any one of the preceding Embodiments,wherein X is I.

Embodiment 23

The scintillation crystal of any one of Embodiments 1 to 21, wherein Xis a combination of I and Br.

Embodiment 24

The scintillation crystal of Embodiment 23, wherein X includes at least50 mol %, at least 70 mol %, or at least 91 mol % I.

Embodiment 25

The scintillation crystal of any one of the preceding Embodiments,wherein for radiation in a range of 300 nm to 700 nm, the scintillationcrystal has an emission maximum at a wavelength of at least 400 nm, atleast 405 nm, or at least 410 nm when the scintillation crystal isexposed to gamma radiation having an energy of 60 keV.

Embodiment 26

The scintillation crystal of any one of the preceding Embodiments,wherein for radiation in a range of 300 nm to 700 nm, the scintillationcrystal has an emission maximum at a wavelength no greater than 430 nm,no greater than 425 nm, or no greater than 420 nm when the scintillationcrystal is exposed to gamma radiation having an energy of 60 keV.

Embodiment 27

The scintillation crystal of any one of the preceding Embodiments,wherein for radiation in a range of 300 nm to 700 nm, the scintillationcrystal has an emission maximum at a wavelength in a range of 400 nm to430 nm, 405 nm to 425 nm, or 410 nm to 420 nm when the scintillationcrystal is exposed to gamma radiation having an energy of 60 keV.

Embodiment 28

The scintillation crystal of any one of the preceding Embodiments,wherein the scintillation crystal has an energy resolution less than6.4%, less than 6.2%, less than 6.0%, or less than 5.5% when measured at662 keV, 22° C., and an integration time of 1 microsecond.

Embodiment 29

The scintillation crystal of any one of the preceding Embodiments,wherein the scintillation crystal has an energy resolution less than6.1%, less than 6.0%, or less than 5.9% when measured at 662 keV, 22°C., and any integration time in a range of 1 microsecond to 4microseconds.

Embodiment 30

The scintillation crystal of any one of the preceding Embodiments,wherein at energies in the range of 32 keV to 81 keV, the scintillationcrystal has an average relative light yield as normalized to a lightyield at 2615 keV of at least 1.01, at least 1.04, or at least 1.07.

Embodiment 31

The scintillation crystal of any one of the preceding Embodiments,wherein at energies in the range of 32 keV to 81 keV, the scintillationcrystal has an average relative light yield as normalized to a lightyield at 2615 keV of no greater than 1.15, no greater than 1.13, or nogreater than 1.11.

Embodiment 32

The scintillation crystal of any one of the preceding Embodiments,wherein at energies in the range of 32 keV to 81 keV, the scintillationcrystal has an average relative light yield as normalized to a lightyield at 2615 keV that is in a range of 1.01 to 1.15, 1.04 to 1.13, or1.07 to 1.11.

Embodiment 33

The scintillation crystal of any one of the preceding Embodiments,wherein at energies in the range of 122 keV to 511 keV, thescintillation crystal has an average relative light yield as normalizedto a light yield at 2615 keV of at least 1.01, at least 1.02, or atleast 1.03.

Embodiment 34

The scintillation crystal of any one of the preceding Embodiments,wherein at energies in the range of 122 keV to 511 keV, thescintillation crystal has an average relative light yield as normalizedto a light yield at 2615 keV no greater than 1.07, no greater than 1.06,or no greater than 1.05.

Embodiment 35

The scintillation crystal of any one of the preceding Embodiments,wherein at energies in the range of 122 keV to 511 keV, thescintillation crystal has an average relative light yield as normalizedto a light yield at 2615 keV that is in a range of 1.01 to 1.07, 1.02 to1.06, or 1.03 to 1.05.

Embodiment 36

The scintillation crystal of any one of Embodiments 30 to 35, whereinthe relative light yield at a particular energy (in units of keV), asnormalized to the light yield at 2615 keV, is:

${{{relative}\mspace{14mu} {light}\mspace{14mu} {yield}} = \frac{{actual}\mspace{14mu} {light}\mspace{14mu} {yield}}{{predicted}\mspace{14mu} {light}\mspace{14mu} {yield}}},$

where the actual light yield is at the particular energy, and thepredicted light yield is:

${{{predicted}\mspace{14mu} {light}\mspace{14mu} {yield}} = {\frac{{particular}\mspace{14mu} {energy}}{2615\mspace{14mu} {keV}} \times {LY}_{2615}}},$

where LY₂₆₁₅ is the actual light yield at 2615 keV.

Embodiment 37

The scintillation crystal of any one of Embodiments 30 to 36, whereinthe average relative light yield is an integral of the relative lightyield over a particular energy range divided by the particular energyrange.

Embodiment 38

The scintillation crystal of any one of the preceding Embodiments,wherein the scintillation crystal has a pulse decay time that is atleast 5%, at least 11%, or at least 20% less than a pulse decay time aNaI:Tl crystal when the scintillation crystal and the NaI:Tl crystal aremeasured at 22° C. and exposed to gamma radiation having an energy of662 keV.

Embodiment 39

The scintillation crystal of any one of the preceding Embodiments,wherein the scintillation crystal has a pulse decay time that is nogreater than 80%, no greater than 65%, or no greater than 50% less thana pulse decay time of NaI:Tl crystal when the scintillation crystal andthe NaI:Tl crystal are measured at 22° C. and exposed to gamma radiationhaving an energy of 662 keV.

Embodiment 40

The scintillation crystal of any one of the preceding Embodiments,wherein the scintillation crystal has a pulse decay time that is in arange of 5% to 80%, 11% to 65%, or 20% to 50% less than a pulse decay ofa NaI:Tl crystal when the scintillation crystal and the NaI:Tl crystalare measured at 22° C. and exposed to gamma radiation having an energyof 662 keV.

Embodiment 41

The scintillation crystal of any one of the preceding Embodiments,wherein the scintillation crystal is monocrystalline.

Embodiment 42

A radiation detection apparatus comprising:

the scintillation crystal of any one of the preceding Embodiments; and

a photosensor optically coupled to the scintillation crystal.

Embodiment 43

The radiation detection apparatus of Embodiment 42, further comprising awindow disposed between the scintillation crystal and the photosensor;

Embodiment 44

The radiation detection apparatus of Embodiment 42 or 43, furthercomprising a clear adhesive attached to a surface of the scintillationcrystal closest to the photosensor.

Embodiment 45

The radiation detection apparatus of any one of Embodiments 42 to 44,wherein the radiation detection apparatus is configured to performprompt gamma neutron activation analysis.

Embodiment 46

A method comprising:

providing the scintillation crystal of any one of the precedingEmbodiments;

capturing radiation within the scintillation crystal;

determining a pulse decay time and an actual light yield of theradiation captured;

determining an estimated light yield corresponding to the pulse decaytime; and

calculating an adjusted light yield that is a product of the actuallight yield times the light yield of NaX:Tl divided by the estimatedlight yield.

EXAMPLES

The concepts described herein will be further described in the Examples,which do not limit the scope of the invention described in the claims.The Examples demonstrate performance of scintillation crystals ofdifferent compositions. Numerical values as disclosed in this Examplessection may be averaged from a plurality of readings, approximated, orrounded off for convenience. Samples were formed using a verticalBridgman crystal growing technique.

Scintillation crystals were formed to compare light yield (PH) andenergy resolution (PHR) of the co-doped samples to a NaI:Tl standard.The compositions of the scintillation crystals are listed in Table 1.Testing was performed at room temperature (approximately 22° C.) byexposing the scintillation crystals to ¹³⁷Cs and using a photomultipliertube and multichannel analyzer to obtain a spectrum.

TABLE 1 Standard, Sr, and La Compositions SrI₂ Sample T1I (mol %) (mol%) LaI₃ (mol %) NaI:T1 (standard) 0.1000 — — NaI:T1, Sr 0.05  0.11 —NaI:T1, La 0.0880 — 8.6 × 10⁻⁵

FIGS. 4 and 5 include spectra that compare the NaI:Tl standard to the Srand La co-doped samples. Energy resolution was performed with a 1microsecond integration time. As can be seen in FIG. 4, the Sr co-dopedsample has significantly better energy resolution of about 5.3%, ascompared to the NaI:Tl standard, which is approximately 6.5%. The Laco-doped sample has light yield that is about 109% of the light yield ofthe NaI:Tl standard (i.e., 9% more light yield).

Ca co-doped samples were also compared to the NaI:Tl standard. Table 2includes the crystal compositions. The Ca #1, Ca #2, and Ca #3 samplescorrespond to locations near the top, middle, and bottom, respectively,of the Ca co-doped crystal. Testing was performed at room temperature(approximately 22° C.) by exposing the scintillation crystals to ¹³⁷Csand using a photomultiplier tube and multichannel analyzer to obtain aspectrum.

TABLE 2 Standard and Ca Compositions CaI₂ Sample T1I (mol %) (mol %)NaI:T1 (standard) 0.100 — NaI:T1, Ca #1 0.06  0.09 NaI:T1, Ca #2 0.03 0.21 NaI:T1, Ca #3 0.07  0.47

Table 3 includes light yield (PH) and energy resolution (PHR) for thesamples when using integration times of 1, 2, and 4 microseconds. ForPH, light yield of the Ca co-doped samples were compared to the lightyield of the NaI:Tl standard.

TABLE 3 PH and PHR for Ca Co-Doped Samples Sample PH (%) PHR (%) NaI:T1100 6.4% NaI:T1, Ca #1  78 5.9% NaI:T1, Ca #2  83 5.4% NaI:T1, Ca #3  885.6%

The best pulse height resolution with the Ca co-doped sample is 5.4%.Further work may be performed to determine whether the Ca concentration,relative Ca to Tl concentrations, or both have a significant impact onthe energy resolution. For example, a Ca concentration of at 0.09 mol %to 0.47 mol % or a ratio of Ca:Tl of 1.5 to 7.0 may provide betterenergy resolution for a Ca co-doped sample as compared to another Caco-doped sample outside either or both of the ranges. Further studiescan help to provide a better insight as to effects of concentrations onenergy resolution.

Proportionality was tested by comparing the Sr and Ca co-doped samplesto the NaI:Tl standard. Table 4 includes the crystal compositions. TheSr #1, Sr #2, and Sr #3 samples correspond to locations near the top,middle, and bottom, respectively, of the Sr co-doped crystal.

TABLE 4 Standard, Sr, and Ca Compositions SrI2 Sample T1I (mol %) (mol%) CaI₂ (mol %) NaI:T1 (standard) 0.100 — — NaI:T1, Sr #1 0.05  0.11  —NaI:T1, Sr #2 0.029 0.135 — NaI:T1, Sr #3 0.030 0.137 — NaI:T1, Ca 0.053— 0.020

For each of the samples, light yield data was collected at a pluralityof different energies by exposing the samples to different radiationsources that emit different energies. Each sample was normalized to thelight yield at 2615 keV for the same sample. The relative light yield ata particular energy (in units of keV), as normalized to the light yieldat 2615 keV, is:

${{{relative}\mspace{14mu} {light}\mspace{14mu} {yield}} = \frac{{actual}\mspace{14mu} {light}\mspace{14mu} {yield}}{{predicted}\mspace{14mu} {light}\mspace{14mu} {yield}}},$

where the actual light yield is at the particular energy, and thepredicted light yield is:

${{{predicted}\mspace{14mu} {light}\mspace{14mu} {yield}} = {\frac{{particular}\mspace{14mu} {energy}}{2615\mspace{14mu} {keV}} \times {LY}_{2615}}},$

where LY₂₆₁₅ is the actual light yield at 2615 keV. Ideally, the plotsshould have all points at 1.00. The average relative light yield wasobtained by integrating the relatively light yield over a particularenergy range to obtain an integrated value, and dividing the integratedvalue by the particular energy range.

FIG. 6 includes a plot of relative light yield as a function of energyfor the samples. As can be seen in FIG. 6, proportionality becomes worseas the energy decreases. The Sr co-doped samples have the bestproportionality as compared to the Ca co-doped and standard samples. Theimprovement is very apparent at low energy. The average light yield forthe Sr co-doped samples is approximately 1.09 at energies in a range of32 keV to 81 keV. For the same energy range, the Ca-doped sample(approximately 1.15) showed improvement over the standard sample(approximately 1.16). An improvement can still be seen at intermediateenergies. For energies in the range of 122 keV to 511 keV, the Srco-doped samples had a relatively light yield of approximately 1.04, theCa co-doped sample had a relatively light yield of approximately 1.06,the standard sample had a relatively light yield of approximately 1.07.

Single crystals from melts that included NaI, Tl at 0.1 atomic % withrespect to Na, with or without co-doping. When co-doped, Ca²⁺ waspresent at 0.1, 0.3, and 0.6 atomic %, and Sr²⁺ was present at 0.05,0.1, 0.2, and 0.4 atomic %. Crystals formed from the melts had thecompositions as listed below in Table 5.

TABLE 5 Crystal Compositions Co-doping^(a) [TI⁺]^(b) [Sr²⁺]^(b) or[Ca²⁺]^(b) TI⁺ only 0.08 ± 0.03% 0 0.05% Sr²⁺ 0.08 ± 0.05% 0.05 ± 0.02%0.1% Sr²⁺ 0.05 ± 0.02% 0.11 ± 0.01% 0.2% Sr²⁺ 0.06 ± 0.04% 0.18 ± 0.01%0.4% Sr²⁺ 0.09 ± 0.06% 0.52 ± 0.08% 0.1% Ca²⁺ 0.06 ± 0.03% 0.09 ± 0.03%0.3% Ca²⁺ 0.03 ± 0.01% 0.21 ± 0.03% 0.6% Ca²⁺ 0.07 ± 0.02% 0.47 ± 0.02%^(a)atomic % in the melt, with respect to Na⁺. ^(b)Measured in growncrystal with inductively coupled plasma—Optical emission spectrometry(ICP-OES); in atomic %.

The crystals were tested for decay times, which were determined byfitting averaged traces corresponding to 662 keV photopeaks withexponential decay functions. Thirty traces were averaged for eachmeasurement. Scintillation pulses were fitted with double exponentialdecay functions. A summary of fast and slow decay times is listed inTable 6.

TABLE 6 Decay Components for Crystals τ_(secondary) Co-dopingτ_(primary) (ns) (ns) TI⁺ only 220 ± 10^(a) (96%)^(b) 1500 ± 200 (4%)0.05% Sr²⁺ 201 ± 21 (94%)   860 ± 240 (6%) 0.1% Sr²⁺ 172 ± 10 (92%)  860 ± 160 (8%) 0.2% Sr²⁺ 195 ± 16 (96%)  690 ± 90 (4%) 0.4% Sr²⁺ 195 ±7 (96%)   1000 ± 300 (4%) 0.1% Ca²⁺ 199 ± 10 (95%)  1030 ± 150 (5%) 0.3%Ca²⁺ 173 ± 12 (94%)   830 ± 230 (6%) 0.6% Ca²⁺ 186 ± 11 (94%)   870 ±110 (6%) ^(a)Uncertainties are the deviations of measured results ofsamples from the same crystal ingot. ^(b)The values in the parenthesisare the percentage of total scintillation light in the specific decaycomponent. Uncertainties are 1-2%.

On average, co-doped crystals with 0.1% Sr²⁺ and 0.3% Ca²⁺ show theshortest decay among their peers. Both show an exceptionally fastprimary decay time of about 170 ns, which is over 20% faster than thatof standard NaI:Tl⁺. The fastest decay recorded is for a sample from the0.3% Ca²⁺ co-doped crystal. The sample shows decay times of 155 ns(92%)+530 ns (8%).

Note that not all of the activities described above in the generaldescription or the examples are required, that a portion of a specificactivity may not be required, and that one or more further activitiesmay be performed in addition to those described. Still further, theorder in which activities are listed is not necessarily the order inwhich they are performed.

Certain features that are, for clarity, described herein in the contextof separate embodiments, may also be provided in combination in a singleembodiment. Conversely, various features that are, for brevity,described in the context of a single embodiment, may also be providedseparately or in any subcombination. Further, reference to values statedin ranges includes each and every value within that range.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any feature(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature of any or all the claims.

The specification and illustrations of the embodiments described hereinare intended to provide a general understanding of the structure of thevarious embodiments. The specification and illustrations are notintended to serve as an exhaustive and comprehensive description of allof the elements and features of apparatus and systems that use thestructures or methods described herein. Separate embodiments may also beprovided in combination in a single embodiment, and conversely, variousfeatures that are, for brevity, described in the context of a singleembodiment, may also be provided separately or in any subcombination.Further, reference to values stated in ranges includes each and everyvalue within that range. Many other embodiments may be apparent toskilled artisans only after reading this specification. Otherembodiments may be used and derived from the disclosure, such that astructural substitution, logical substitution, or another change may bemade without departing from the scope of the disclosure. Accordingly,the disclosure is to be regarded as illustrative rather thanrestrictive.

What is claimed is:
 1. A well logging device, comprising: ascintillation crystal in the well logging device, the scintillationcrystal comprising NaX:Tl, Me²⁺, and RE, wherein: X represents ahalogen; Me²⁺ represents a divalent metal element; RE represents a rareearth element; TI has a dopant concentration in a range of 1×10⁻⁵ mol %to 0.2 mol %; Me²⁺ has a dopant concentration greater than a dopantconcentration of Tl; and the scintillation crystal has a pulse decaytime that is at least 5% less than a pulse decay time a NaI:Tl crystalwhen the scintillation crystal and the NaI:Tl crystal are measured at22° C. and exposed to gamma radiation having an energy of 662 keV. 2.The well logging device of claim 1, wherein RE has a concentration of atleast 1×10⁻⁴ mol %.
 3. The well logging device of claim 1, wherein X isa combination of I and Br.
 4. The well logging device of claim 1,wherein X is I.
 5. The well logging device of claim 1, wherein RE is Sc,Y, Lu, Yb, or any combination thereof.
 6. The well logging device ofclaim 1, wherein the scintillation crystal has an emission maximum at awavelength no greater than 430 nm.
 7. A medical imaging device,comprising: a scintillation crystal in the medical imaging device, thescintillation crystal comprising NaX:Tl, Me²⁺, wherein: X represents ahalogen; Me²⁺ represents a divalent metal element; and TI has a dopantconcentration in a range of 1×10⁻⁵ mol % to 0.2 mol %; wherein thescintillation crystal has a property including: an emission maximum at awavelength no greater than 430 nm.
 8. The medical imaging device ofclaim 7, wherein Me²⁺ is Sr having a concentration no greater than 0.9mol %.
 9. The medical imaging device of claim 7, wherein thescintillation crystal has a greater light yield as compared to a NaI:Tlcrystal when the scintillation crystal and the NaI:Tl crystal aremeasured at 22° C.
 10. The medical imaging device of claim 7, whereinthe scintillation crystal has an energy resolution less than 6.4% whenmeasured at 662 keV, 22° C., and an integration time of 1 microsecond.11. The medical imaging device of claim 8, wherein Sr has aconcentration in a range of 0.01 mol % to 0.2 mol %.
 12. The medicalimaging device of claim 7, wherein at energies in the range of 32 keV to81 keV, the scintillation crystal has an average relative light yield asnormalized to a light yield at 2615 keV of no greater than 1.15.
 13. Themedical imaging device of claim 7, wherein Me²⁺ is Ca having aconcentration no greater than 0.9 mol %.
 14. The medical imaging deviceof claim 13, wherein Ca has a concentration in a range of 0.01 mol % to0.2 mol %.
 15. A medical imaging device, comprising: a scintillationcrystal in the medical imaging device, the scintillation crystalcomprising NaX:Tl, Me, wherein: X represents a halogen; Me represents aGroup 1 element, a Group 2 element, or any combination thereof; TI has adopant concentration in a range of 1×10⁻⁵ mol % to 0.2 mol %; and Me hasa dopant concentration greater than the dopant concentration of Tl; andthe scintillation crystal has a property including: an energy resolutionless than 6.4% when measured at 662 keV, 22° C., and an integration timeof 1 microsecond; or a pulse decay time that is less than anotherscintillation crystal that has a composition of NaX:Tl.
 16. The medicalimaging device of claim 15, wherein the scintillation crystal has anenergy resolution less than 6.4% when measured at 662 keV, 22° C., andan integration time of 1 microsecond.
 17. The medical imaging device ofclaim 15, wherein at energies in the range of 32 keV to 81 keV, thescintillation crystal has an average relative light yield as normalizedto a light yield at 2615 keV of no greater than 1.15.
 18. The medicalimaging device of claim 15, wherein at energies in the range of 122 keVto 511 keV, the scintillation crystal has an average relative lightyield as normalized to a light yield at 2615 keV no greater than 1.07.19. The medical imaging device of claim 14, wherein Me has aconcentration in a range of 0.01 mol % to 0.9 mol %.
 20. The medicalimaging device of claim 19, wherein Tl has a concentration in a range of1×10⁻⁴ mol % to 0.2 mol %.